1. Field of the Invention
This invention pertains to the field of diagnostic assays for detecting infection of an animal by the protozoan parasite Giardia lamblia. 
2. Background
Giardia is a protozoan parasite that is a major cause of diarrhea worldwide. The most common species of Giardia is G. lamblia, which is the most common pathogenic parasite in North America (Meyer and Jarrol (1980) Am. J. Epidemiol. 3: 1-12). Giardia has two life stages. The trophozoite stage inhabits the small intestine of host animals, moving about using a flagella. A suction disk allows the trophozoite to attach to the wall of the intestine while it feeds on mucous secretions. The second life stage, the cyst, has a stronger outer layer, and thus better able than the trophozoite to survive outside of the host while passing from host to host. Transmission is typically through Giardia-contaminated water supplies (Meyer and Jarrol, supra.), or person to person (Black et al. (1977) Pediatrics 60: 486-491).
The cytoskeleton of G. lamblia trophozoites contain a group of 29-38 kDa proteins known as giardins (Peattie et al. (1989) J. Cell Biol. 109: 2323-2335). Nucleic acid sequences are known for several of the giardins, including α-1-giardin and α-2-giardin, which are 81% identical at the nucleic acid level and have amino acid sequences that are 77% identical (Alonso and Peattie (1992) Mol. Biochem. Parasitol. 50: 95-104). The α-1-giardin has been identified on the membrane and disk of G. lamblia trophozoites (Wenman et al. (1993) Parasitol. Res. 79: 587-592).
Traditionally, Giardia infection is diagnosed by microscopic detection of ova and parasites (O&P) in stools, which is a laborious process. More recently developed methods for Giardia diagnosis include serologic tests for anti-Giardia antibodies. Little correlation was found, however, between the presence of anti-Giardia antibodies in the serum and active Giardia infection. Other diagnostic methods involve detection of Giardia antigens in stool samples. For example, Green et al. discuss the use of an affinity-purified antiserum raised by inoculating rabbits with whole trophozoites or disrupted trophozoites and cysts (Green et al. (1985) Lancet 2: 691-693). Other groups have described the use of monospecific antibodies that bind to a 65 kDa antigen that is shed in the stool of giardiasis patients (Rosoff and Stibbs (1986) J. Clin. Microbiol. 24: 1079-1083; U.S. Pat. No. 5,503,983; Stibbs (1989) J. Clin. Microbiol. 27: 2582-2588; Rosoff et al. (1989) J. Clin. Microbiol. 27: 1997-2002). Monoclonal antibodies that bind to two species of Giardia cyst wall constituents are discussed in Lujan et al. (1995) J. Biol. Chem. 270: 29307-29313. ELISA assays for G. lamblia are discussed in, for example, Nash et al. (1987) J. Clin. Microbiol. 25: 1169-1171; Stibbs et al. (1988) J. Clin. Microbiol. 26: 1665-1669; Ungar et al. (1984) J. Infect. Dis. 149: 90-97.
Previously described assays for detecting Giardia infection often have shortcomings. For example, the assay of Ungar et al. was reported to fail to detect 8% of positive samples, and cannot be read by direct visual inspection (Green et al., supra.). Therefore, a need exists for improved methods for detecting Giardia infection in animals, including humans. The present invention fulfills this and other needs.